Monotube strut with rebound cut-off feature

ABSTRACT

A suspension damper including a tube and a damping piston assembly disposed within the tube and slidably mounted therein for reciprocal movement in the tube. The suspension damper also includes a piston rod extending through the tube and connected to the damping piston assembly, a rod guide assembly closing a bottom end of the tube, and a rebound cut-off disk and spring suspended in the tube between the rod guide assembly and the damping piston assembly and cooperating with the damping piston assembly to provide a rebound cut-off effect between the rebound cut-off disk and the damping piston assembly. The rebound cut-off disk and spring have specific gravities that are greater than the specific gravity of the fluid in the tube, such that the disk and the spring sink in the fluid.

TECHNICAL FIELD

[0001] The present invention relates to a suspension damper with reboundcut-off for use in a vehicle suspension system and, more particularly,to a suspension damper with a hydraulic rebound cut-off feature thatprovides a hydraulically cushioned stop at an end of rebound travel inthe damper.

BACKGROUND OF THE INVENTION

[0002] A damper operates in vehicle suspensions as a damping devicecontrolling the sprung (body) and unsprung (wheels) masses of a vehicleby reducing loads or vertical accelerations normally transmitted fromthe wheels to the body. Damping is accomplished by converting kineticenergy into thermal energy and dissipating the heat. Conventionally,hydraulic dampers include a piston with a connected piston rod slidablycarried in a fluid-filled tube and separating the tube into extensionand compression chambers. A rod guide at the top end of the tube closesthe extension chamber and slidably engages the piston rod. As the partsof the vehicle suspension to which the cylinder tube and piston rod areattached move relative to one another, the damping piston assembly ismoved in compression and rebound strokes along the axis of the damper.The damping piston assembly includes passages, and in some dampers alsospecial valve arrangements associated with the passages, that allowfluid in the working chamber to flow through the damping piston assemblyat a controlled rate to provide damping of the relative motion betweenthe parts of the vehicle suspension to which the damper is attached.

[0003] In many applications, the suspension damper is called upon tolimit the full extension travel of the suspension system. In some priordampers, mechanical rebound stops that are fixed to the piston rod andengageable with the rod guide are known to provide a means of limitingthe maximum extension travel of the piston rod from the damper. Atypical mechanical rebound stop is generally equipped with a resilientbumper made of material such as rubber or urethane. The bumper isdesigned to cushion the engagement of the damping piston with the rodguide at the end of damper travel in the extension direction. This typeof a mechanical stop tends to result in somewhat of an abrupt means oflimiting travel during rebound. It has also been found that, in severeapplications, a resilient bumper material may undesirably experienceheat degradation when the bumper absorbs the entire rebound stop load.

[0004] It has also been the practice in some prior hydraulic dampers toprovide elements attached to the damping piston assembly and thecylinder tube that provide additional hydraulic damping force actingagainst the piston during a portion of the rebound stroke, for slowingthe damping piston assembly as it approaches the end of the reboundstroke. This function of providing additional damping at the end of therebound stroke, for slowing the rate of rebound, is also known ashydraulic “rebound cut-off.” Examples of this approach are disclosed inU.S. Pat. No. 6,209,691 to Fehring et al. and U.S. Pat. No. 5,706,920 toPees et al., and in British Patent No. 691,477 to Stephens.

[0005] In recent years, hydraulic dampers using a special type of fluid,known as Magneto-Rheological (MR) fluid, have been utilized as part ofvehicle traction and stability enhancement control systems for activelycontrolling the amount of damping provided under varying road andoperating conditions to provide improved performance and safe operationof vehicles. An MR fluid is generally significantly more viscous and hasa higher specific gravity than the hydraulic fluids used in priorvehicle dampers. As a result, elements for providing a hydraulic reboundcut-off function in prior hydraulic dampers, or spacers attached to thedamping piston assembly and/or piston rod for limiting maximum extensionor speed of extension of the damper on the rebound stroke, may provideinefficient and undesirable performance in dampers using MR fluids orother fluids.

[0006] Providing a hydraulic rebound cut-off feature with a shockabsorber form of damper is known. Such a device is disclosed in U.S.Pat. No. 2,379,750. That hydraulic rebound cut-off feature uses a rodguide having a collar forming an anchorage for an upper end of a coilspring whose lower end is secured to a ring valve. When the pistonapproaches full extension, the ring valve is contacted, which closessome fluid passages completely and others partially to reduce theirfluid flow capacity, increasing damping force and slowing extensiondirected travel. This prior art device undesirably restricts fluid flowbetween the valve and the piston.

[0007] Yet another type of known hydraulic rebound cut-off featureutilizes a rebound cut-off piston in conjunction with a damping piston.Such a device is disclosed in U.S. Pat. No. 4,342,447. According to thisprior art design, a fixed/clamped disk or disk stack on a secondary orrebound cut-off piston co-acts with the damping piston to effect asubstantial entrapment of fluid in the extension chamber of the shockabsorber as the damping piston approaches full rebound. However, in thisdevice, an indentation in the wall acts as a piston stop and not as asupport for the rebound cut-off piston.

[0008] Still another type of known hydraulic rebound cut-off featureutilizes a rebound cut-off device in conjunction with a damping piston.Such a device is disclosed in U.S. Pat. No. 5,277,284. According to thisprior art design, a spring is held on the damping piston by a retainingring on the piston rod. However, in this device, the retaining ring doesnot act as a support for the rebound cut-off device.

[0009] In other dampers, the rebound cut-off disk is retained by a snapring in the wall of the tube, or is attached to the rod guide by meansof a spring. However, the snap ring reduces available stroke, while theattachment is awkward and prone to failure. Still other dampers includea rebound cut-off disk that is heavier than fluid in the tube, and theyare supported by a plastic float. The float requires a certain volumeand, therefore, takes up valuable stroke length. Moreover, in a monotubestrut, the reservoir is upside down, so the float will not stay near therod guide.

[0010] Particularly with monotube design dampers, maximizing activelength is critical. This is because a typical monotube damper carries agas cup that separates out a gas chamber within the single tube of thedevice. The gas chamber is expansible and contractible to account forthe changing volume of space occupied by the piston rod entering andexiting the tube. Presence of the gas chamber minimizes the amount ofactive length that can be utilized by other features such as the reboundcut-off device. Therefore, there is a need in the art for a reboundcut-off feature for a monotube damper with minimal impact on damper deadlength.

SUMMARY OF THE INVENTION

[0011] According to the present invention, an improved hydraulic reboundcut-off feature of a suspension damper is provided. It can be added to adamper with minimal impact on damper dead length, has fewer parts, islow cost and light weight.

[0012] According to this aspect, a suspension damper includes afluid-filled tube and a damping piston assembly disposed within the tubeand slidably mounted therein for reciprocal movement in the tube. Apiston rod extends through the tube and connects to the damping pistonassembly, and a rod guide assembly closes a bottom end of the tube. Thesuspension damper also includes a rebound cut-off disk suspended in thetube between the rod guide assembly and the damping piston assembly. Thesuspension damper further includes a spring suspended in the tubebetween the rod guide assembly and the rebound cut-off disk. The springand the disk have specific gravities greater than the specific gravityof the fluid in the tube such that they sink to the bottom of the tubeadjacent the rod guide assembly. Together, the spring and the reboundcut-off disk cooperate with the damping piston assembly to provide arebound cut-off effect between the rebound cut-off disk and the dampingpiston assembly.

[0013] The invention may also take the form of a method for limiting theextent and/or speed of extension of the damper on the rebound stroke byhaving a spring and a disk with specific gravities less than thespecific gravity of the liquid in the tube to provide a rebound cut-offeffect between the rebound cut-off disk and the damping piston assembly.

[0014] A suspension damper of the present invention incorporates ahydraulic rebound cut-off feature. As a result, a suspension damper ofthe present invention is light-weight, has fewer parts and is relativelylow cost. The rebound cut-off effect results in the full damper areahaving a very high force, yet the compressing loads are essentiallyzero.

[0015] Other features and advantages of the present invention will bereadily appreciated as the same becomes better understood after readingthe subsequent description, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:

[0017]FIG. 1 is a schematic cross-section of a monotube strut of thepresent invention.

[0018]FIG. 2 is a schematic cross-section of the suspension damperhoused in the monotube strut of FIG. 1.

DETAILED DESCRIPTION

[0019]FIG. 1 illustrates an exemplary embodiment of a suspension damper10 of the present invention, housed in a monotube strut. It may beunderstood that the suspension damper 10 of the present invention may behoused in any variation of a monotube strut known in the industry.

[0020]FIG. 2 depicts an exemplary embodiment of a suspension damper 10,according to the present invention, having a cylinder tube 12 defining aworking chamber 14 for containing a fluid therein, and defining an axis16. A rod guide assembly 18 closes a bottom end of the cylinder tube 12,and is adapted for receiving and guiding a piston rod 20, and forproviding a sliding fluid seal between the piston rod 20 and thecylinder tube 12.

[0021] A damping piston assembly 22 is slidably disposed in the workingchamber 14 for reciprocating motion along the axis 16. The piston rod 20has a first end 24 and a second end 26. The first end 24 of the pistonrod 20 is connected to the damping piston assembly 22, for linearmovement of the piston rod 20 and damping piston assembly 22 within theworking chamber 14, along the axis 16. The second end 26 of the pistonrod 20 extends along the axis 16, through the rod guide assembly 18, andout of the working chamber 14.

[0022] Those having skill in the art will recognize that the componentsdescribed thus far, in relation to FIG. 2, would further include anumber of features known in the art, such as fluid passages and valvecomponents in the damping piston assembly, for example, that have beenomitted from this explanation for clarity. We contemplate, however, thatour invention may be practiced in forms incorporating such featuresknown to those having skill in the art.

[0023] The damper 10 further includes a rebound cut-off feature, in theform of a disk 28 having a central bore 30 therein, whereby the disk 28is disposed about the piston rod 20 between the damping piston assembly22 and the rod guide 18 for sliding motion of the disk 28 within thecylinder tube 12 and along the piston rod 22. The cylinder tube 12defines an inner surface 13 thereof, and the disk 28 defines an outersurface 29 thereof, adjacent to the inner surface 13 of the cylindertube 12. The outer surface 29 is spaced from the inner surface 13 of thecylinder tube 12 to form a small gap 31 between the cylinder tube 12 andthe disk 28, thereby defining a generally annular-shaped restriction inthe fluid passage 33, in conjunction with the inner wall 13 of thecylindrical tube 12. The axial length and outer periphery of the reboundcut-off disk 28 can be closely controlled and matched to the inner wall13 of the cylinder tube 12 to accurately and conveniently control theeffective flow characteristics of the fluid passage 33, and facilitatetuning of the damper 10 to meet desired performance parameters forrebound cut-off operation.

[0024] The working chamber 14 includes a volume of fluid 32 having aspecific gravity. The disk 28 is configured in such a manner that thespecific gravity of the disk 28 is greater than the specific gravity ofthe fluid, so that the disk 28 sinks in the fluid 32 toward the bottomend of the working chamber 14, adjacent the rod guide assembly 18,during normal operation of the damper 10.

[0025] In the first exemplary embodiment, the fluid 32 is aMagneto-Rheological (MR) fluid, but other fluids such as oil are alsocontemplated. The disk 28 is fabricated as a solid circular disk, havinga specific gravity greater than that of the fluid. In an exemplaryembodiment, the disk is made of metal, and is over-molded with a plasticmaterial or elastomeric material, such as nylon, such that the specificgravity of the resulting component is more than that of the fluid 32.The elastomer serves to reduce the noise when it impacts the piston.Such materials are robust enough to absorb the physical loads imposed onthe disk 28 in arresting the motion of the damping piston assembly 22,and heavy enough to sink in the fluid 32. It may be understood, however,that the materials for the disk 28 may be varied depending on the typeof fluid 32 used in the working chamber 14, whether an MR fluid or othertype of fluid.

[0026] Because the disk 28, in a damper 10 according to the presentinvention, sinks in the fluid 32 to the bottom of the working chamber14, away from the damping piston assembly 22, rather than having aspacer attached to the damping piston assembly or piston rod as in priordampers, the fluid 32 does not need to flow around the disk 28 duringnormal, i.e., non-maximum, extension of the damper 10. This allows thegap 31 between the disk 28 and the cylinder tube 12 to be smaller thanwould be the case in prior dampers, even when using highly viscous MRfluids.

[0027] Having a small gap 31 between the cylinder tube 12 and the disk28 results in a tighter fit between the cylinder tube 12 and the disk 28that is conducive to reducing operational noise of the damper 10, suchas rattling of the disk 28 in the cylinder tube 12. Having the disk 28float in the working chamber 14 away from the damping piston 22 alsocontributes to reducing operational noise.

[0028] The damper 10 of FIG. 2 further includes a conical compressionspring 36. The working chamber 14 includes a volume of fluid 32 having aspecific gravity. The spring 36 is configured in such a manner that thespecific gravity of the spring 36 is greater than the specific gravityof the fluid, so that the spring 36 sinks in the fluid 32 to the bottomend of the working chamber 14 during normal operation of the damper 10.The spring 36 is disposed between the rod guide assembly 18 and the disk28 for holding the disk 28 away from the rod guide assembly 18, andforming a small reservoir 38 of fluid 32 between the rod guide assembly18 and disk 28. The spring 36 can be fabricated from any material suchthat the specific gravity of the spring 36 is greater than that of thefluid 32.

[0029] As discussed above, the gap 31 between the cylinder tube 12 andthe disk 28 defines a passage 33, which permits fluid flow between thedisk 28 and the inner surface 13 of the cylinder tube 12 for filling andemptying the small reservoir 38 created by the spring 36 between thedisk 28 and the rod guide assembly 18.

[0030] The material and configuration of the disk 28, spring 36, andfluid 32 are selected to have respective specific gravities that resultin the disk 28 and spring 36 being sinkable in the fluid 32 at thebottom end of the working chamber 14.

[0031] In another embodiment of the present invention, the disk 28 has aspecific gravity sufficiently greater than the specific gravity of thefluid 32 such that the disk 28 and the spring 36 will sink in the fluid32. Thus, in this embodiment, the specific gravity of the spring 36 doesnot necessarily have to be greater than that of the fluid 32, and may infact, be equal to or greater than the specific gravity of the fluid.

[0032] During normal, i.e., non-rebound-cut-off, operation of the damper10, the damping piston assembly 22 is free to move independently fromthe disk 28 and its related components, as the piston rod 20 moves inand out of the cylinder tube 12 in response to the motion of the vehiclesuspension. As the damper 10 approaches a maximum extended position onthe rebound stroke of the damper 10, the damping piston assembly 22 isbrought to bear against the disk 28.

[0033] Further motion of the damping piston assembly 22 on the reboundstroke, after the damping piston assembly 22 contacts the disk 28,forces the disk 28 to move downward, compressing the spring 36 such thatthe coils of the spring nest inside each other, due to the conicalconfiguration, thereby having a minimum collapsed length of about onewire diameter. The compression of the spring 36 forces fluid trappedbelow the disk 28 in the small reservoir 38 to flow through the passage33 past the rebound cut-off disk 28 in a controlled manner thatgenerates additional damping which resists and slows the damping pistonassembly 22 to thereby provide a hydraulic rebound cut-off function thatreduces impact of the damping piston 22 against the rod guide 18. Oncethe spring's 36 compression is maximized, rebound cut-off operationceases and the disk 28 functions as a spacer, preventing furtherextension of the damper 10 during the rebound stroke.

[0034] Following rebound cut-off operation, the spring 36 forces thedisk 28 away from the rod guide 18, as the damping piston assembly 22moves upward along the axis 16 on the compression stroke of the damper10. As the disk 28 moves away from the rod guide 18, fluid 32 in theworking chamber 14 flows through the passage 33 and past the disk 28 torefill the small reservoir 38.

[0035] While the present invention has been illustrated by thedescription of one or more embodiments thereof, and while theembodiments have been described in considerable detail, they are notintended to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications willreadily appear to those skilled in the art. The invention in its broaderaspects is therefore not limited to the specific details, representativeapparatus and method and illustrative examples shown and described.Accordingly, departures may be made from such details without departingfrom the scope or spirit of Applicant's general inventive concept.

What is claimed is:
 1. A monotube strut assembly having a suspensiondamper comprising: a tube; a rod guide assembly closing a bottom end ofthe tube; a damping piston assembly disposed within the tube andslidably mounted therein for reciprocal movement in the tube, whereinthe tube is substantially filled with a fluid having a specific gravitythat damps the reciprocating movement of the damping piston assemblywithin the tube; a piston rod connected to the damping piston assemblyand extending through the tube and the rod guide assembly; a reboundcut-off disk suspended in the tube between the rod guide assembly andthe damping piston assembly and cooperating with the damping pistonassembly to provide a rebound cut-off effect between the rebound cut-offdisk and damping piston assembly; and a spring disposed between therebound cut-off disk and the rod guide assembly, wherein the disk has aspecific gravity that is greater than the specific gravity of the fluid,whereby the disk sinks in the fluid.
 2. The monotube strut assembly ofclaim 1 wherein the spring has a specific gravity that is greater thanthe specific gravity of the fluid, whereby the spring sinks in thefluid.
 3. The monotube strut assembly of claim 1 wherein the fluid is amagneto-rheological (MR) fluid.
 4. The monotube strut assembly of claim1 wherein the fluid is oil.
 5. The monotube strut assembly of claim 1wherein the disk comprises metal overmolded with a plastic material. 6.The monotube strut assembly of claim 1 wherein the disk comprises metalovermolded with an elastomeric material.
 7. The monotube strut assemblyof claim 1 wherein the spring is conical in shape and the coils of thespring nest within each other, thereby having a minimum collapsed lengthof about one wire diameter.
 8. The monotube strut assembly of claim 1wherein the disk is circular in shape and has an outer peripherydefining an annular gap in conjunction with an inner wall of the tubefor fluid flow therebetween.
 9. The monotube strut assembly of claim 1wherein the spring has a specific gravity equal to or less than thespecific gravity of the fluid and the disk has a specific gravity thatis sufficiently greater than the specific gravity of the fluid such thatthe spring and the disk sink in the fluid and remain near the rod guideassembly.
 10. A monotube strut assembly having a suspension dampercomprising: a tube; a rod guide assembly closing a bottom end of thetube; a damping piston assembly disposed within the tube and slidablymounted therein for reciprocal movement in the tube, wherein the tube issubstantially filled with a fluid having a specific gravity that dampsthe reciprocating movement of the damping piston assembly within thetube; a piston rod connected to the damping piston assembly andextending through the tube and the rod guide assembly; a rebound cut-offdisk suspended in the tube between the rod guide assembly and thedamping piston assembly and cooperating with the damping piston assemblyto provide a rebound cut-off effect between the rebound cut-off disk anddamping piston assembly; and a spring disposed between the reboundcut-off disk and the rod guide assembly, wherein the disk has a specificgravity that is greater than the specific gravity of the fluid, wherebythe disk sinks in the fluid, wherein the disk comprises metal overmoldedwith a plastic material or an elastomeric material, wherein the disk iscircular in shape and has an outer periphery defining an annular gap inconjunction with an inner wall of the tube for fluid flow therebetween,wherein the spring has a specific gravity that is greater than thespecific gravity of the fluid, whereby the spring sinks in the fluid,and wherein the spring is conical in shape and the coils of the springnest within each other, thereby having a minimum collapsed length ofabout one wire diameter.
 11. The monotube strut assembly of claim 10wherein the fluid is a magneto-rheological (MR) fluid.
 12. The monotubestrut assembly of claim 10 wherein the fluid is oil.
 13. The monotubestrut assembly of claim 10 wherein the spring has a specific gravityequal to or less than the specific gravity of the fluid and the disk hasa specific gravity that is sufficiently greater than the specificgravity of the fluid such that the spring and the disk sink in the fluidand remain near the rod guide assembly.